Soil pH is a measurement that quantifies the acidity or alkalinity of the soil, representing the concentration of hydrogen ions (H+) in the soil solution. The scale ranges from 0 to 14, with a value of 7 considered neutral; numbers below 7 indicate increasing acidity, and numbers above 7 indicate increasing alkalinity. Because the pH scale is logarithmic, a change of just one unit represents a tenfold difference in acidity; for example, a soil with a pH of 5 is ten times more acidic than one with a pH of 6. This chemical property governs the reactions and biological processes that determine plant health.
Controlling Nutrient Accessibility
Soil pH dictates the solubility of minerals and the chemical forms of nutrients, controlling their availability for plant uptake. Most common garden and crop plants thrive in a slightly acidic to neutral range, typically between pH 6.0 and 7.0. Within this range, the majority of essential macronutrients like Nitrogen, Phosphorus, and Potassium, as well as secondary nutrients like Calcium and Magnesium, are optimally available to plant roots.
When the soil becomes too acidic, falling below a pH of 5.5, elements such as Aluminum and Manganese increase dramatically in solubility, often reaching concentrations that are toxic to plant roots. Simultaneously, the availability of Phosphorus, Calcium, and Magnesium declines, leading to severe deficiencies that stunt growth. Phosphorus is especially sensitive, with its maximum availability peaking in the slightly acidic range of pH 5.5 to 6.5.
Conversely, when the soil becomes too alkaline, generally above pH 7.5, the availability of several micronutrients is reduced. Iron, Zinc, Copper, and Manganese become inaccessible to plants even if they are physically present in high total amounts. Plants growing in these high-pH soils often display distinct yellowing of new growth, a classic sign of iron or zinc deficiency.
Influence on Soil Microbes and Texture
Soil pH directly influences the composition and activity of microorganisms that drive soil fertility and structure. Bacteria are generally favored by slightly acidic to alkaline conditions, thriving best in the pH range of 6.0 to 8.0. Fungi, however, are more tolerant of acidic environments and often dominate the microbial population in soils with a pH below 5.5.
One of the most impacted biological functions is nitrification, the two-step process in which bacteria convert ammonium (NH4+) into the nitrate (NO3-) form of nitrogen. This conversion is most rapid in soils with a pH between 7.0 and 8.0, and the activity of the responsible bacteria can slow to near zero when the pH drops below 5.0. This reduction in bacterial activity directly impedes the nitrogen cycle, limiting the amount of nitrate available for plant growth.
Soil pH extremes also negatively affect larger soil organisms that contribute to soil structure and texture. Earthworms, which are instrumental in aeration and creating stable soil aggregates, prefer a neutral to slightly alkaline environment, typically between pH 6.5 and 7.5. In highly acidic soils below pH 5.5, earthworm populations decline, which reduces their beneficial impact on organic matter decomposition and soil structure. Mycorrhizal fungi can be impacted by pH extremes.
Testing and Modifying Soil Acidity
Understanding the current pH of a soil is the first step in effective management, and this is typically done through testing. While home soil kits are widely available and offer quick results, they generally provide only an approximate pH value and are often inaccurate for quantifying specific nutrient levels. Professional laboratory analysis is far more accurate, providing a precise pH reading and a buffer pH, which measures the soil’s resistance to change. This lab report also provides specific recommendations for correcting the soil’s acidity.
To raise the pH of acidic soil, a process known as liming, the most common materials are agricultural lime or dolomitic lime. The carbonate in the lime neutralizes the excess hydrogen ions in the soil, raising the pH to the desired range. This process is not instantaneous; it relies on the dissolution of the material and can take several months to a year, especially in soils with a high buffering capacity.
To lower the pH of alkaline soil, amendments like elemental sulfur are applied. Soil bacteria slowly convert the elemental sulfur into sulfuric acid, which then lowers the soil pH. Because this is a biologically mediated process, it is slow and can take up to two years to fully realize the desired effect. In soils with high calcium carbonate, lowering the pH often requires impractical and expensive application rates of sulfur.